My mom converted about $40,000 of her traditional IRA into a Roth IRA at the end of 2017.
In the beginning of 2018, she figured that wasn't a good idea, so she undid the Roth conversion by doing a recharacterization.
She got a 1099-R for the conversion. Is there another tax form for the recharacterization so it cancels out the 1099-R?
Clearly, she does not want to report the 1099-R as income on her tax return. That was the whole point of the recharacterization: It should be as if the conversion never happened in the first place.
She is married filing jointly for tax year 2017, and should be filing single in tax year 2018.

My mom converted about $40,000 of her traditional IRA into a Roth IRA at the end of 2017.
In the beginning of 2018, she figured that wasn't a good idea, so she undid the Roth conversion by doing a recharacterization.
She got a 1099-R for the conversion. Is there another tax form for the recharacterization so it cancels out the 1099-R?
Clearly, she does not want to report the 1099-R as income on her tax return. That was the whole point of the recharacterization: It should be as if the conversion never happened in the first place.
She is married filing jointly for tax year 2017, and should be filing single in tax year 2018.

Yes, there will be a 1099R reporting the recharacterization, but it will not be issued until Jan, 2019. Therefore, while she will not report the conversion on her 2017 return, she WILL have to include an explanatory statement indicating the date and amount of the conversion, date and amount of the recharaterization and what the conversion was worth when transferred back to the TIRA in January. This will explain to the IRS why she is not reporting conversion income on her 2017 joint return.

Because the 1099R for the recharacterization will be coded R to reflect that a 2017 conversion was recharacterized, she will not have to report any of this on her 2018 single return. Of course, she should check that 1099R to be sure it is correct and the code is correct.

1,200 shares of VTHRX (Vanguard Target Retirement 2030 Fund) was converted from Traditional to Roth IRA in 2017.
The conversion was reversed by a recharacterization in 2018.
1,227.8260 shares of VTHRX was recharacterized. At first glance, this looks about right, since there were annual fund distributions at the end of 2017.
Upon closer inspection, I am not sure how the extra 27.8260 shares came about.

I add up $0.68850 of distributions per share. Multiply that by 1,200 shares,and you get $826.20 of distributions. Divide that by the reinvest price of $33.69, and I get 24.5236 extra shares, not 27.8260.